(Na ion plus K ion) ATPase is the enzyme responsible for the active transport of Na ion and K ion across the plasma membranes of all animal cells. The larger subunit of the enzyme is responsible for this activity. We are cleaving this large subunit with cyanogen bromide and isolating the fragments which result. Sequence information from these, and methionine-containing tryptic peptides, will order them in a linear map. Vesicles which have been purified directly from the kidney and which are either inside-out or right-side-out, will be used to label independently inside surfaces and outside surfaces of the protein; and these labeled side chains will be positioned in the map. Sulfhydrals which respond to conformational changes will be identified as well as amino acid residues between the subunits which form the (alpha beta) 2 dimer. As results from such experiments accumulate it should be possible, in the minds' eye, to coil the protein into its three-dimensional structure.